1. Field of the Invention
The present invention relates to an optical switch that is manufactured by use of micromachining technology and used in changing the optical path of an optical signal, and more particularly, to an optical switch configured such that the electrode-to-electrode distance between a movable plate-like electrode and a fixed or stationary electrode of the optical switch can be easily set to a desired distance.
2. Description of the Related Art
Optical switches of various types that are manufactured by use of micromachining technology have been heretofore provided. For example, an example of the prior art 2×2 optical switch will be described with reference to FIGS. 1 and 2. In the specification, the 2×2 optical switch is defined such that it has two input side optical fibers mounted thereto each emitting an optical signal therefrom and two output side optical fibers mounted thereto each receiving an optical signal incident thereon and that is capable of concurrently switching the optical paths of optical signals emitted respectively from the two output side optical fibers so that each optical signal is incident on corresponding one of the two input side optical fibers.
FIG. 1 is a plan view illustrating a construction of the prior art 2×2 optical switch, and FIG. 2 is a generally sectional view taken along the line 2—2 in FIG. 1 and looking in the direction indicated by the arrows. The illustrated optical switch SW1 comprises: a movable electrode supporting plate 14 of a generally rectangle in plan; an opening 13 of a generally square in plan formed in the movable electrode supporting plate 14 at generally the central portion thereof; four V-shaped grooves 25A, 26A and 25B, 26B, two grooves 25A and 26A of which are formed on the top surface of the movable electrode supporting plate 14 in the longitudinal direction thereof at one side of the opening 13 and the remaining two grooves 25B and 26B of which are formed on the top surface of the movable electrode supporting plate 14 in the longitudinal direction thereof at the opposite side of the opening 13; a fixed or stationary electrode substrate 19 of a generally rectangle in plan and of a substantially the same shape and size as those of the movable electrode supporting plate 14, the fixed electrode substrate 19 being disposed underneath the movable electrode supporting plate 14 and having its elevated portion 18 on the center thereof to be fitted into the opening 13 of the movable electrode supporting plate 14; a movable plate-like electrode 11 of a generally square in plan that is disposed substantially in parallel with the elevated portion 18 of the fixed electrode substrate 19 with a predetermined space or gap between them and above the opening 13 of the movable electrode supporting plate 14; four flexible beams 12A1, 12A2, 12B1, and 12B2 for supporting the movable plate-like electrode 11 to be movable toward and away from the elevated portion 18 of the fixed electrode substrate 19; and four thin plate-like or sheet-like micro-mirrors 17A1, 17A2, 17B1, and 17B2 upstanding from the top surface of the movable plate-like electrode 11.
Each of the four flexible beams 12A1, 12A2, 12B1, and 12B2 is called “flexure” in this technical field, and the base of each of the two beams 12A1 and 12A2 is integrally connected to corresponding one of the two opposed ends of one anchor portion 15A of a generally rectangle in plan in the longitudinal direction thereof, the anchor portion 15A being opposed to the other anchor portion 15B of a generally rectangle in plan. The base of each of the remaining two beams 12B1 and 12B2 thereof is integrally connected to corresponding one of the two opposed ends of the other anchor portion 15B of a generally rectangle in plan in the longitudinal direction thereof. Each of the four beams 12A1, 12A2 and 12B1, 12B2 extends from corresponding one of the two opposed ends of each of the anchor portions 15A and 15B along corresponding one of the four sides of the movable plate-like electrode 11, and each of the four ends thereof is integrally connected to corresponding one of the four corners of the movable plate-like electrode 11. The paired anchor portions 15A and 15B are fixed to the movable electrode supporting plate 14 at the both sides of the opening 13 in the longitudinal direction of the movable electrode supporting plate 14.
Each of the four V-shaped grooves 25A, 26A and 25B, 26B extends from a predetermined position outside the pair of anchor portions 15A and 15B (a predetermined position toward the ends of the movable electrode supporting plate 14 in the longitudinal direction thereof) to corresponding one of the ends of the movable electrode supporting plate 14 in the longitudinal direction thereof, and the two V-shaped grooves 25A and 26A and the two V-shaped grooves 25B and 26B opposed to one another through the opening 13 put therebetween in the longitudinal direction of the movable electrode supporting plate 14 are aligned with one another.
The four thin plate-like micro-mirrors 17A1, 17A2, 17B1 and 17B2 have substantially the same shape and size with one another, and the two micro-mirrors 17A1 and 17B1 are disposed on a first straight line that passes through substantially the center of the movable plate-like electrode 11 and forms an angle of 45° with a horizontal line. The remaining two micro-mirrors 17A2 and 17B2 are disposed on a second straight line that passes through substantially the center of the movable plate-like electrode 11 and is orthogonal to the first straight line. These micro-mirrors 17A1, 17A2, 17B1 and 17B2 are disposed at such positions that they are away by substantially the same distance in the radial direction from the intersection between the first and second straight lines and that axial lines of corresponding two V-shaped grooves 25A and 26B and of corresponding two V-shaped grooves 26A and 25B pass through substantially the centers of the micro-mirrors.
The micro-mirrors 17A1, 17A2, 17B1, 17B2 may be fabricated, for example, by applying a photoresist film of a predetermined thickness on the top surface of the movable plate-like electrode 11, exposing only portions of the photoresist film on which the micro-mirrors are to be formed, thereafter removing the photoresist film except for the exposed portions thereof by use of a solvent to form four micro-mirror bodies, and coating mirror surfaces of these micro-mirror bodies with a metal such as gold (Au) or the like.
Alternatively, as described in Japanese Patent Application No. 243582/2001 (or PCT Application PCT/JP02/08177 filed on Aug. 9, 2002 claiming a priority of Japanese Patent Application No. 243582/2001) filed on Aug. 10, 2001 by the same assignee as that of the present application, micro-mirrors erecting from a silicon substrate may be fabricated by applying an orientation-dependent wet etching or chemical anisotropic wet etching to the silicon substrate the surface of which is (100) crystal face. In such case, since the mirror surface of each micro-mirror becomes (100) crystal face, the accuracy in verticality and flatness of each micro-mirror comes to much high, and hence optical loss can be minimized. Further, the details thereof will be referred to the above Japanese Patent Application No. 243582/2001 (or PCT/JP02/08177), or a paper entitled “SELF ALIGNED VERTICAL MIRRORS AND V-GROOVES APPLIED TO A SELF-LATCHING MATRIX SWITCH FOR OPTICAL NETWORKS” published by Philippe Helin, et al. in Thirteenth IEEE International Micro Electro Mechanical Systems Conference (MEMS-2000) held on Jan. 23 through 27, 2000 at Miyazaki, Japan.
Optical fibers are located and mounted in the four V-shaped grooves 25A, 26A and 25B, 26B, respectively. In this example, an output side optical fiber 31A and an input side optical fiber 32A are located and mounted in the V-shaped grooves 25A and 26A positioned in the left side in FIG. 1, respectively, and in the V-shaped grooves 25B and 26B positioned in the right side in FIG. 1 are located and mounted an output side optical fiber 31B and an input side optical fiber 32B, respectively. As a result, the output side optical fiber 31A mounted in the V-shaped groove 25A and the input side optical fiber 32A mounted in the V-shaped groove 26B are opposed to and aligned to each other (are disposed on the same optical axis line), and the input side optical fiber 32A mounted in the V-shaped groove 26A and the output side optical fiber 31B mounted in the V-shaped grooves 25B are opposed to and aligned to each other (are disposed on the same optical axis line).
The movable electrode supporting plate 14, four beams 12A1, 12A2, 12B1 and 12B2, the pair of anchor portions 15A and 15B, and movable plate-like electrode 11 can be formed into one body. For example, a substrate made of single crystal silicon of a predetermined thickness is used as the movable electrode supporting plate 14, and an insulation layer, for example, silicon dioxide (SiO2) layer 21 is formed on the top surface of the single crystal silicon substrate 14, and on the top surface of the silicon dioxide layer 21 is formed, for example, a single crystal silicon layer. The single crystal silicon layer is processed using photolithography technology to form the above-mentioned four beams 12A1, 12A2, 12B1 and 12B2, the pair of anchor portions 15A and 15B, and movable plate-like electrode 11 therefrom. Thereafter, the single crystal silicon substrate 14 is etched from the bottom surface side thereof to form the opening 13 of a generally square therein. Thus, the four beams 12A1, 12A2, 12B1 and 12B2, the pair of anchor portions 15A and 15B, and movable plate-like electrode 11 are formed into one body as well as the pair of anchor portions 15A and 15B is integrally fixed to the top surface of the movable electrode supporting plate 14. Further, in FIG. 2, a reference numeral 22 denotes an insulation layer (for example, silicon dioxide layer) that is previously formed on the bottom surface of the movable electrode supporting plate 14. This insulation layer 22 is used as a mask in forming the opening 13 in the movable electrode supporting plate 14 by use of photolithography technology.
In general, an SOI (Silicon on Insulator) substrate of a generally rectangle in plan will be used to form the movable electrode supporting plate 14, four beams 12A1, 12A2, 12B1 and 12B2, the pair of anchor portions 15A and 15B, and movable plate-like electrode 11 that are united with one another by use of photolithography technology. Since such manufacturing method for an optical switch is well known, it will be described in brief, here. Further, a 2×2 optical switch having the same construction as that of the above-mentioned 2×2 optical switch has been described in, for example, Japanese Patent Application Public Disclosure No. 82292/2002 (Japanese Patent Application No. 270621/2000), and therefore, the details thereof including the manufacturing method will be referred to this Japanese Patent Application Public Disclosure No. 82292/2002 (P2002-82292A).
First, an SOI substrate of a generally rectangle in plan is prepared. Generally, the SOI substrate is constituted by three layers that are a thick support substrate made of single crystal silicon, an insulation layer on the top of the thick support substrate, and a thin layer of single crystal silicon on the top of the insulation layer. In this example, there is used an SOI substrate comprising a thick support substrate (not shown in FIGS. 1 and 2) of single crystal silicon of a generally rectangle in plan, an insulation layer 21 of silicon dioxide formed on the top surface of the support substrate, and a thin layer (not shown in FIGS. 1 and 2) of single crystal silicon joined onto the top surface of the silicon dioxide layer 21. However, it goes without saying that any SOI substrate manufactured by use of one of known other methods or processes may be used. Further, in this example, the insulation layer (for example, silicon dioxide layer) 22 is previously formed on the bottom surface of the SOI substrate.
Next, by use of photolithography technology, a patterning of the thin single crystal silicon layer of the SOI substrate is done to leave portions thereof corresponding to shapes of the four beams 12A1, 12A2, 12B1 and 12B2, the pair of anchor portions 15A and 15B, and movable plate-like electrode 11 so that the thin single crystal silicon layer is removed except for the portions thereof corresponding to shapes of the four beams, the pair of anchor portions, and movable plate-like electrode. Thereafter, a portion of the silicon dioxide layer 22 on the bottom surface of the SOI substrate, which corresponds to the opening 13 of the movable electrode supporting plate 14, is removed. Thus, the four beams 12A1, 12A2, 12B1 and 12B2, the pair of anchor portions 15A and 15B, and movable plate-like electrode 11 are formed into one body from the thin single crystal silicon layer on the silicon dioxide layer 21 of the SOI substrate.
Next, the support substrate of single crystal silicon is etched from the bottom surface side of the SOI substrate using KOH solution to form the opening 13. As a result, the movable electrode supporting plate 14 of a generally rectangle in plan is formed from the support substrate of single crystal silicon of a generally rectangle in plan.
Further, it is needless to say that the opening 13 of a generally square formed in the movable electrode supporting plate 14 has such a size that it can accommodate the movable plate-like electrode 11 and four beams 12A1, 12A2, 12B1 and 12B2 therein.
The fixed electrode substrate 19 is a substrate made of, for example, single crystal silicon of a generally rectangle in plan, and at the central portion of the top surface thereof is formed the above-mentioned elevated portion 18 having substantially the same shape (in this example, generally square) and size as those of the opening 13 formed through the movable electrode supporting plate 14. The movable electrode supporting plate 14 constructed as discussed above is put on the fixed electrode substrate 19, and then, they are united by using, for example, a suitable adhesive or bonding agent. When the movable electrode supporting plate 14 is put on the fixed electrode substrate 19, the elevated portion 18 of the fixed electrode substrate 19, which serves as a fixed electrode fits into the opening 13 of the movable electrode supporting plate 14 from the bottom side thereof so that the elevated portion 18 of the fixed electrode substrate 19 and the movable plate-like electrode 11 are mounted in opposed state to each other with a predetermined space or gap therebetween. In such manner, the optical switch SW1 shown in FIGS. 1 and 2 is constructed. Further, in FIG. 2, a reference numeral 28 denotes an insulation layer (silicon dioxide layer) that is used as a mask in forming the elevated portion 18 on the fixed electrode substrate 19. This insulation layer 28 prevents the movable plate-like electrode 11 from being electrically connected to the elevated portion 18 of the fixed electrode substrate 19 when the movable plate-like electrode 11 is driven toward the elevated portion 18 and comes into contact with the top surface of the elevated portion 18.
According to the optical switch SW1 constructed as discussed above, the movable plate-like electrode 11 is allowed to move into the opening 13, and if a predetermined drive voltage is applied between the fixed electrode substrate 19 and the movable plate-like electrode 11 to produce an electrostatic attraction therebetween in such direction that the movable plate-like electrode 11 and the fixed electrode substrate 19 are attracted to each other, the movable plate-like electrode 11 is downwardly displaced, and hence the micro-mirrors 17A1, 17A2, 17B1 and 17B2 formed on and upstanding from the top surface of the movable plate-like electrode 11 are also displaced downwardly to a position where the micro-mirrors are out of the optical paths through each of which an optical signal (beam) will propagate.
Specifically explaining, in case any drive voltage is not applied between the fixed electrode substrate 19 and the movable plate-like electrode 11 so that the movable plate-like electrode 11 is not displaced and hence the micro-mirrors on the top surface of the movable plate-like electrode 11 exist on the optical paths through which optical signals (beams) emitted respectively from the output side optical fibers 31A and 31B will propagate, the optical signal (beam) emitted from the output side optical fiber 31A is reflected by the micro-mirror 17A1 existing on the optical path for that optical signal in the direction of forming an angle of 90° (forming a right angle) with the incident beam, and is incident on the micro-mirror 17A2. The incident optical signal is further reflected by the micro-mirror 17A2 in the direction of forming an angle of 90°, and is incident on the input side optical fiber 32A. Likewise, the optical signal (beam) emitted from the output side optical fiber 31B is reflected by the micro-mirror 17B1 existing on the optical path for that optical signal in the direction of forming a right angle with the incident beam, and is incident on the micro-mirror 17B2. The incident optical signal is further reflected by the micro-mirror 17B2 in the direction of forming an angle of 90°, and is incident on the input side optical fiber 32B.
On the contrary, in case a predetermined drive voltage is applied between the fixed electrode substrate 19 and the movable plate-like electrode 11 so that the movable plate-like electrode 11 is electrostatically driven toward the fixed electrode substrate 19 and hence the micro-mirrors on the top surface of the movable plate-like electrode 11 are moved downwardly so that they do not exist on (they are out of) the optical paths through which optical signals (beams) emitted respectively from the output side optical fibers 31A and 31B will propagate, the optical signal emitted from the output side optical fiber 31A goes right on without being reflected by the micro-mirrors 17A1 and 17B2, and is incident on the input side optical fiber 32B that is opposed to the output side optical fiber 31A. Likewise, the optical signal emitted from the output side optical fiber 31B goes right on without being reflected by the micro-mirrors 17B1 and 17A2, and is incident on the input side optical fiber 32A that is opposed to the output side optical fiber 31B. Thus, the optical path for the optical signal emitted from the output side optical fiber 31A can be switched from the input side optical fiber 32A to the input side optical fiber 32B, and similarly, the optical path for the optical signal emitted from the output side optical fiber 31B can be switched from the input side optical fiber 32B to the input side optical fiber 32A. In other words, the optical switch SW1 constructed as described above is capable of switching in space the optical path of an optical signal propagating through an optical waveguide or optical transmission line (path) without any intervention of a solid state optical waveguide.
In an optical switch of this type, in general, it is desirable in circuit design to make it possible that the movable plate-like electrode 11 is driven toward the stationary electrode substrate 19 by application of a drive voltage as low as possible. In other words, it is desirable in circuit design to reduce the absolute value of a drive voltage for driving the movable plate-like electrode 11 toward the stationary electrode substrate 19 to a value as small as possible. For such reason, in the prior art optical switch SW1 constructed as discussed above, the movable plate-like electrode 11, the four beams 12A1, 12A2, 12B1, 12B2, and the anchor portions 15A, 15B are formed into one body, and yet, the thickness of the movable plate-like electrode 11 and the four beams 12A1, 12A2, 12B1, 12B2 is made thin to lighten the weight thereof and to lessen the elastic forces of the four beams. However, the four beams must have their elastic forces for holding the movable plate-like electrode 11 substantially in parallel with the fixed electrode substrate 19 and for returning the movable plate-like electrode 11 already attracted to the fixed electrode substrate 19 to its original position therefrom. Therefore, there is a limit in reducing the magnitude of the drive voltage that is required to drive the movable plate-like electrode 11 by the predetermined distance toward the fixed electrode substrate 19.
In addition, in the above-constructed optical switch SW1, there is proposed another method for reducing the absolute value of a drive voltage in which the electrode-to-electrode distance (gap) between the movable plate-like electrode 11 and the fixed electrode substrate 19 is set to a necessary and minimum length. In such case, the thickness of the movable electrode supporting plate 14 (in case of using an SOI substrate, the support substrate of single crystal silicon) cannot be made thin too much in view of manufacturing process, and accordingly, there is adopted a procedure that the electrode-to-electrode distance between the movable plate-like electrode 11 and the fixed electrode substrate 19 is set to a necessary and minimum distance by providing the elevated portion 18 on the surface of the fixed electrode substrate 19 as shown in FIG. 2.
In this manner, in case the electrode-to-electrode distance is set to a necessary and minimum distance, the movable plate-like electrode 11 is electrostatically driven toward the fixed electrode substrate 19 until it comes into contact with the top surface of the elevated portion 18.
The relationship between the drive voltage to be applied to the movable plate-like electrode 11 and the distance that the movable plate-like electrode 11 is to be driven is not linear. It is characterized in that when the drive voltage applied to the movable plate-like electrode 11 is gradually increased, the movable plate-like electrode 11 is driven downwardly toward the fixed electrode substrate 19, and that when the driven distance of the movable plate-like electrode 11 becomes equal to or more than ⅓ of the electrode-to-electrode distance X between the bottom surface of the movable plate-like electrode 11 and the top surface of the elevated portion 18 of the fixed electrode substrate 19, the movable plate-like electrode 11 is driven at a dash toward the fixed electrode substrate 19 and is attracted or stuck to the top surface of the fixed electrode substrate 19 (in practice, the top surface of the elevated portion 18). A drive voltage by which the movable plate-like electrode 11 is driven at a dash toward the fixed electrode substrate 19 is called “pull-in voltage” in this technical field. Further, the details of the pull-in voltage will be referred to Japanese Patent Application Nos. 75443/2002 (P2002-75443) and 75817/2002 (P2002-75817) both of which were filed on Mar. 19, 2002, by the same assignee as that of the present application, or the homepage of Professor Hiroshi TOSHIYOSHI, Institute of Industrial Science, University of Tokyo:    http://toshi.fujita3.iis.u-tokyo.ac.jp/onlinelecture/electrostatic1.pdf
In the prior art, since the movable plate-like electrode 11 is driven until it comes into contact with the top surface of the elevated portion 18 of the fixed electrode substrate 19 (by the distance X), the drive voltage is necessarily set to a voltage equal to or higher than the pull-in voltage. For this reason, when the movable plate-like electrode 11 moves over the distance equal to ⅓ of the electrode-to-electrode distance X, it is driven at a dash toward the fixed electrode substrate 19 thereby to come into contact with the top surface of the elevated portion 18 of the fixed electrode substrate 19.
When the movable plate-like electrode 11 is displaced downwardly and the bottom surface thereof comes into contact with the top surface of the elevated portion 18 of the fixed electrode substrate 19, a phenomenon occurs that van der Waals' force acts or affects between the bottom surface of the movable plate-like electrode 11 and the top surface of the elevated portion 18 of the fixed electrode substrate 19 so that they are attracted to each other, and that the movable plate-like electrode 11 is not restored to its original position in an instant even the application of the drive voltage is stopped. That is, there occurs a phenomenon that the movable plate-like electrode 11 and the elevated portion 18 of the fixed electrode substrate 19 are temporarily or permanently attracted or stuck to each other by van der Waals' force. This phenomenon is called “sticking” in this technical field. Consequently, it is impossible to switch the path of an optical signal at once, and hence there is a disadvantage that the reliability of switching operation is greatly deteriorated.
In view of the foregoing, there is proposed an optical switch that is constructed such that minute protrusions are formed on either one of the bottom surface of the movable plate-like electrode 11 opposed to the fixed electrode substrate 19 or the top surface of the fixed electrode substrate 19 opposed to the movable plate-like electrode 11 to reduce the contact area between the movable plate-like electrode 11 and the fixed electrode substrate 19, or a second fixed electrode pattern for preventing the sticking from occurring is formed on the top surface of the fixed electrode substrate 19, thereby to prevent occurrence of the phenomenon that the movable plate-like electrode 11 and the fixed electrode substrate 19 are temporarily or permanently attracted to each other by van der Waals' force. Such optical switch is disclosed in, for example, Japanese Patent Application Public Disclosure No. 256563/1998, Japanese Patent Application Public Disclosure No. 264650/2001 (P2001-264650A), or Japanese Patent Application Public Disclosure No. 39392/2003 (P2003-39392A).
There is shown in FIG. 3 an example of the prior art optical switch in which a plurality of minute protrusions are formed on the top surface of the elevated portion 18 of the fixed electrode substrate 19 to reduce the contact area between the movable plate-like electrode 11 and the elevated portion 18 of the fixed electrode substrate 19. FIG. 3 is a sectional view similar to FIG. 2, and the optical switch SW2 shown in FIG. 3 may have the same construction, structure and shape as those of the prior art optical switch SW1 already discussed with reference to FIGS. 1 and 2 except that a plurality of minute protrusions 23 are formed, for example, in a matrix manner on the top surface of the elevated portion 18 of the fixed electrode substrate 19. Therefore, in FIG. 3, portions and elements corresponding to those shown in FIGS. 1 and 2 will be denoted by the same reference characters attached thereto and explanation thereof will be omitted unless necessary.
In case a plurality of minute or very small protrusions 23 are formed, for example, in a matrix manner on the top surface of the elevated portion 18 of the fixed electrode substrate 19, when the movable plate-like electrode 11 is electrostatically driven downwardly and comes into contact with the top surface of the elevated portion 18 of the fixed electrode substrate 19, the bottom surface of the movable plate-like electrode 11 comes into contact with the pointed ends of these protrusions 23, and hence the movable plate-like electrode 11 is not in surface contact with the elevated portion 18 of the fixed electrode substrate 19. Accordingly, it is possible to greatly decrease the contact area between them and to prevent the sticking from occurring.
However, in order to prevent the movable plate-like electrode 11 from being electrically connected to the elevated portion 18 of the fixed electrode substrate 19 when the movable plate-like electrode 11 is driven toward the fixed electrode substrate 19 and comes into contact with the elevated portion 18 thereof, the protrusions 23 must be made of an insulation material or the surfaces of the protrusions 23 must be covered or coated with an insulation material. In case of forming the plural protrusions 23 on the top surface of the elevated portion 18 of the fixed electrode substrate 19, in general, a photoresist is applied on the top surface of the oxide film (oxide film, usually, silicon dioxide layer, used as a mask when the elevated portion 18 is formed) on the elevated portion 18, and the oxide film is patterned in a pattern corresponding to the protrusions 23, and then, the elevated portion 18 is etched using the patterned oxide film as a mask. Thus, the plural protrusions 23 are formed on the top surface of the elevated portion 18. For example, in case a plurality of the fixed electrode substrates 19 each having such elevated portion 18 formed thereon are arranged on a wafer or chip, even if a photoresist is dropped on the top surface of each of the elevated portions 18 and is coated thereon by spinning the wafer, it is impossible to spread the photoresist on the top surface of each elevated portion 18 with uniform thickness and satisfaction. Accordingly, there is a drawback that it is much difficult to form the protrusions 23 with high precision that have been accurately located at desired positions and have uniform height. In other words, in case the fixed electrode substrate 19 has its elevated portion 18 formed on the top surface thereof, it is much difficult to form on the top surface of the elevated portion 18 a plurality of the very small protrusions 23 with high precision that have been accurately located at desired positions and have uniform height. Further, in FIG. 3, a reference character 29 denotes an insulation layer (oxide film) formed on the top surface of the elevated portion 18. In addition, the details of a method of manufacturing a plurality of protrusions will be referred to the above-mentioned Japanese Patent Application Public Disclosure No. 256563/1998, Japanese Patent Application Public Disclosure No. 264650/2001 (P2001-264650A), or Japanese Patent Application Public Disclosure No. 39392/2003 (P2003-39392A).